132 



BIOLOGICAL LECTURES. 



cranial nerves have begun to develop and show definite relations 

 with some of the neuromeres of the hind-brain. For example, 

 as is best shown in Fig. 7, the fifth nerve is connected with the 

 first and second neuromeres of the hind-brain, that is, with the 

 segments Nos. 6 and 7. The eighth neuromere has no nerve 

 connection ; the seventh and eighth 

 nerves are connected with the ninth 

 and tenth neuromeres ; the ninth nerve 

 with the eleventh neuromere, and the 

 front root of the vagus is connected 

 with the twelfth segment. 



The evidence of a primitive head 

 segmentation, which is so well preserved 

 in these animals, is by no means ex- 

 ceptional, as may be determined by 

 examining the embryos of other animals. 

 They are present at least in corre- 

 spondingly young stages of birds and 

 amphibia, and this considerable range 

 indicates they are a fundamental char- 

 acteristic. 



In the very young chick, I have 

 repeatedly examined them in living 

 specimens. They are to be faintly 

 seen as early as the twelfth to the 

 fifteenth hour of incubation ; and from 

 that time onwards they are ever present 

 till they are obliterated by transforma- 

 tions in the brain. In the earlier 

 stages in which I have observed them 

 there are only three pairs, and they 

 apparently increase in number by backward growth. Fig. 8 

 shows the appearance of these segments in a chick embryo 

 while the neural groove is open. The segments extend from 

 the anterior limit of the head as far back as the neural folds are 

 established ; there are here four protovertebrae, but lying in 

 front of them and entirely distinct from them are eleven pairs 

 of the primitive neural segments. 



FIG. 8. Embryo of chick, with 

 open neural groove and three 

 well-marked mesoblastic so- 

 mites. The neural folds are 

 segmented throughout their 

 extent. 



